The little-studied cluster Berkeley 90 – III. Cluster parameters

Abstract

The open cluster Berkeley 90 is the home to one of the most massive binary systems in the Galaxy, LS III +46 • 11, formed by two identical, very massive stars (O3.5 If* + O3.5 If*), and a second early-O system (LS III +46 • 12 with an O4.5 IV((f)) component at least). Stars with spectral types earlier than O4 are very scarce in the Milky Way, with no more than 20 examples. The formation of such massive stars is still an open question today, and thus the study of the environments where the most massive stars are found can shed some light on this topic. To this aim, we determine the properties and characterize the population of Berkeley 90 using optical, near-infrared and WISE photometry and optical spectroscopy. This is the first determination of these parameters with accuracy. We find a distance of 3.5 +0.5 −0.5 kpc and a maximum age of 3 Ma. The cluster mass is around 1 000 M (perhaps reaching 1 500 M if the surrounding population is added), and we do not detect candidate runaway stars in the area. There is a second population of young stars to the Southeast of the cluster that may have formed at the same time or slightly later, with some evidence for low-activity ongoing star formation.

The Villafranca catalog of Galactic OB groups

“…The Gaia DR2 distance to Berkeley 90 of ∼3 kpc is somewhat longer than the two early estimates by Mayer & Macák (1973) and Motch et al (1997) but consistent with the more recent values of Maíz Apellániz et al (2015a) and Marco & Negueruela (2017), especially the former. As with most groups in this paper, our distance agrees with the Cantat-Gaudin et al ( 2018) value but with a much larger uncertainty caused by the covariance term.…”

“…The Gaia DR2 parallax of LS III +46 12 is compatible with that of the cluster, which according to the discussion in Maíz Apellániz et al (2015a) indicates that the star is overluminous for its spectral type or that it is a yet undetected binary. We confirm that the B8 III star 2MASS J20352201+4651518, the B9 III star 2MASS J20352097+4648368, the F6 V star 2MASS J20350745+4651367, the F6 III star 2MASS J20351813+ 4650525, the F8 IV star 2MASS J20350955+4652199, and the G2 III star 2MASS J20351026+4651364 are foreground objects according to their parallaxes and have proper motions significantly different to that of the cluster (Marco & Negueruela 2017).…”

“…From its parallax, proper motion, and CMD position, the B0.5 V star 2MASS J20351422+4650118 (Marco & Negueruela 2017) appears to be either a background object or to have been ejected towards the south after an interaction with LS III +46 11. We also detect four additional possible objects ejected from the cluster, all with G magnitudes that would correspond to B-type stars.…”

The Villafranca catalog of Galactic OB groups

“…In this group it is worth mentioning three stars. 2MASS J20350798+4649321 was classified as O8 V by Marco & Negueruela (2017), who noted that its IPHAS photometry indicated Hα emission. In our blue violet spectrum (Fig.…”

High-mass stars in Milky Way clusters

“…If we take all the stars with these spectral types in the GOSC v3 (Maíz Apellániz et al 2013), we find 20 objects. Of these, 14 lie inside or close to some of the most massive clusters in the Milky Way, but four are in smaller clusters (M cl ∼ 10 3 M ) and two appear in relative isolation within active star formation regions (Marco & Negueruela 2017). Taken at face value, this distribution seems to support the stochastic scenario, but we must exercise caution when interpreting such data.…”